Many apparatus and methods exist for sensing and determining an orientation of an object in three dimensional space. Such apparatus and methods may be used for surveying, mapping, navigation and other purposes.
The word “orientation” refers to the position of an object with respect to some reference point or coordinate system. For example, azimuthal orientation typically refers to horizontal deviation of an object with respect to a standard reference direction such as north or south. Inclination orientation typically refers to vertical deviation of an object with respect to a standard reference direction such as gravity.
Orientation may also be expressed with respect to a direction or coordinate system which is referenced to the object itself. For example, in drilling and surveying of oil and gas wells, it is usual to refer to the longitudinal axis of a pipe string as the “z-axis” and to refer to the plane which is transverse to the pipe string and the z-axis as the “x-y plane”, regardless of the azimuthal orientation or the inclination orientation of the pipe string and its components.
In this coordinate system, the orientation of the z-axis of the pipe string relative to north is often referred to as the “azimuth” of the pipe string and is typically expressed in degrees from true north or magnetic north, the orientation of the z-axis of the pipe string relative to gravity is often referred to as the “inclination” of the pipe string and is typically expressed in degrees from vertical (gravity), and the orientation of the pipe string relative to the x-y plane is often referred to as the “toolface” of the pipe string or of one of its components and is typically expressed in degrees from the “high side” or “low side” of the pipe string.
More particularly, in the case of “toolface” the orientation is typically expressed in degrees from high side or low side of the pipe string with respect to a reference position or “home position” associated with the pipe string.
Measuring instruments for use in determining orientation must provide an appropriate reference against which the orientation is measured. For example, azimuthal orientation may be measured using magnetic reference instruments such as magnetometers, which use the earth's magnetic field as the reference, or inertial reference instruments such as gyroscopes, which use a predetermined base orientation as the reference. Inclination orientation may be measured using gravity reference instruments such as accelerometers, which use the earth's gravitational field as the reference. Toolface orientation which is expressed relative to high side or low side may also be measured using gravity reference instruments such as accelerometers. Each type of measuring instrument has inherent limitations.
For example, magnetic reference instruments are generally unaffected by external forces exerted on the instruments due to movement such as vibration, but magnetic reference instruments are typically unsuitable for use in proximity with iron, steel and other magnetic materials, since local magnetic fields in such materials will distort magnetic measurements. Magnetic reference instruments are also ineffective for determining inclination orientation.
Inertial reference instruments can provide a relatively accurate measurement of both azimuthal orientation and inclination orientation and are not affected by local magnetic fields, but inertial reference instruments tend to be relatively fragile and can be affected by external forces exerted on the instruments, which can distort inertial measurements. Inertial reference instruments are therefore typically unsuitable for use in harsh environments which include excessive heat, pressure and vibration.
Gravity reference instruments are relatively robust and reliable and can be used in proximity with magnetic materials such as steel pipe. However, gravity reference instruments are affected by external forces exerted on the instruments, which forces can distort the measurements obtained by gravity reference instruments. As a result, the use of gravity reference instruments is often avoided in environments where significant and/or random movement of the instruments can be expected.
Magnetic reference instruments, inertial reference instruments and gravity reference instruments are all used for a wide range of applications in many different industries, including the oil and gas industry.
Due to their characteristics and limitations, inertial reference instruments are often used in wellbore surveying and logging applications but are seldom used in drilling applications.
Magnetic reference instruments and gravity reference instruments are both used extensively in drilling applications as components of a measurement-while-drilling survey system. In such a system, magnetic reference instruments are used to measure azimuthal orientation of the drill string, while gravity reference instruments are used to measure inclination orientation and toolface orientation of the drill string. The magnetic reference instruments are typically isolated from local magnetic fields by being mounted in non-magnetic lengths of drilling pipe.
Ideally, both the magnetic reference instruments and the gravity reference instruments are located in a non-rotating section of the drill string so that the measurements are not affected by rotation of the drill string. In sliding drilling applications where the drill bit is rotated by a power source such as a motor and the drill string is non-rotating, the instruments may be located in any section of the non-rotating drill string. In rotary drilling applications where the drill bit is rotated at least in part by rotation of the drill string, rotation of the drill string may be stopped temporarily in order to facilitate orientation measurements.
Alternatively, the instruments may be located in a non-rotating housing mounted on the drill string so that the instruments do not rotate while the drill string rotates. Such a non-rotating housing is often an integral component of a rotary steerable drilling assembly, so that the housing can function as both a steering mechanism for the drill bit and a mounting location for the orientation instruments. Although this configuration is attractive for facilitating orientation measurements, it is in practice relatively difficult to transmit data pertaining to the orientation measurements up the drill string to the surface, since the data must cross over from the non-rotating housing to the rotating drill string in order to be transmitted up the drill string by a measurement-while-drilling telemetry system.
As a result, it would be preferable in applications involving a rotating drill string to avoid locating the orientation instruments on a separate non-rotating housing. Since magnetic reference instruments are not directly affected by movement of the drill string, the incorporation of magnetic reference instruments in a rotating drill string is relatively straightforward. However, since gravity reference instruments are affected by any movement of the drill string and by the resulting forces that may be exerted on the gravity reference instruments, the incorporation of gravity reference instruments in a rotating drill string is relatively complicated, particularly where the gravity reference instruments are to be relied upon for determining inclination orientation and toolface orientation of the drill string.
Attempts have been made to incorporate gravity reference instruments into rotating pipe strings or drill strings.
U.S. Pat. No. 4,958,125 (Jardine et al) describes a method and apparatus for determining the instantaneous rotation speed of a drill string in a borehole by measuring the centripetal acceleration of the drill string at least two opposite ends of a drill string diameter. The centripetal acceleration is measured in opposite directions at each end of the drill string diameter so that the effects of lateral shock or lateral vibration of the drill string can be eliminated from the centripetal acceleration measurements and the value of the centripetal acceleration is used to calculate the rotation speed of the drill string. In an alternate embodiment, the centripetal acceleration of the drill string at the four opposite ends of two perpendicular drill string diameters in the same cross-section of the drill string is measured so as to obtain four values for the centripetal acceleration, from which the centripetal acceleration can be eliminated and the amplitude and direction of the lateral shock can be derived. Jardine et al does not describe or suggest using accelerometers to determine inclination orientation or toolface orientation of a rotating member.
U.S. Pat. No. 6,065,219 (Murphey et al) describes a method and apparatus for estimating the cross-sectional shape and orientation of a borehole and the motion of a tool therein. In its simplest form, the apparatus in Murphey et al is comprised of a rotatable tool which has a plurality of distance sensors such as acoustic calipers, mechanical calipers or electrical resistance sensors for generating standoff signals representative of standoff distances from each of the distance sensors to respective points on the borehole at a plurality of measurement times and at least one angle sensor such as a magnetometer, inclinometer, accelerometer or gyroscope for generating sinusoidal rotational orientation signals representative of the rotational orientation angle of the tool with respect to a reference direction at the plurality of measurement times. Preferably the tool is comprised of at least one gravity-type sensor and at least one magnetic-type sensor so that a satisfactory angle signal is acquired for any orientation of the axis of the tool. The apparatus is further comprised of a signal processor for calculating an estimate of the actual cross-sectional shape and orientation of the borehole based upon the standoff signals and the rotational orientation signals.
In the preferred embodiment, the rotatable tool in Murphey et al is further comprised of a plurality of accelerometers (preferably four) for generating raw acceleration signals proportional to the lateral translational acceleration of the tool, similar to the invention described in Jardine et al. The raw acceleration signals are filtered to eliminate the contribution of gravity to the raw acceleration signals, thus producing filtered acceleration signals which are processed to provide derived displacement signals which are representative of the lateral translational displacement of the tool in the borehole, which derived displacement signals are compared with measured displacement signals obtained from the standoff signals in order to generate error signals which are representative of a primary error function relating to the difference between the estimated shape of the borehole and the actual shape of the borehole.
Murphey et al does not describe or suggest correcting the orientation signals from the angle sensor or sensors to account for the effects of movement of the tool. Murphey et al also does not describe or suggest obtaining an indication of toolface orientation of the tool described in Murphey et al.
There remains a need for an orientation sensing apparatus and for a method for determining an orientation which uses one or more gravity reference instruments and which can be used in an environment where the gravity reference instruments may be subjected to rotation.